In the continuous casting of steel, molten metal is cast directly into thin strip by a casting machine. The shape of the strip is determined by the mold of the casting machine, which receives the molten metal from a tundish and casts the metal into a generally thin strip. The strip may be further subjected to cooling and processing upon exit from the casting rolls.
In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving casting roll surfaces, and are brought together at the nip between the casting rolls to produce a thin cast strip product. The thin cast strip is delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the casting rolls so as to restrain the two ends of the casting pool.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will suffer very rapid scaling due to oxidation at such high temperatures. A sealed enclosure that contains an atmosphere that inhibits oxidation of the strip is therefore provided beneath the casting rolls to receive the thin cast strip, and through which the strip passes away from the strip caster.
The length of a casting campaign of a twin roll caster has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Therefore, the focus of attention in the casting has been to extend the life cycle of the core nozzle, tundish and side dams, and thereby reducing the cost per ton of casting thin strip. When a nozzle, tundish or side dam wears to the point that one of them has to be replaced, the casting campaign has to be stopped, and the worn out component replaced. This generally involves replacing other unworn components as well, otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components. Graphite alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for the side dams, tundish and core nozzle components. Since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel, there is considerable waste of casting time between campaigns. See U.S. Pat. Nos. 5,184,668 and 5,277,243.
The side dams wear independently of the core nozzles and tundish, and independently of each other. During casting the side dams are initially urged against the ends of the casting rolls under applied forces, and “bedded in” by wear so as to ensure adequate seating against outflow of molten steel from the casting pool. The forces applied to the side dams are then reduced after an initial bedding-in period, however there is significant wear of the side dams throughout the casting operation. The core nozzle and tundish components in the metal delivery system usually have a longer potential life than the side dams, and could normally continue in service through several more ladles of molten steel if the useful life of the side dams could be extended. However, the tundish and core nozzle components, which still have useful life, are changed when the side dams are changed to increase the production capacity of the caster.
Previously, each side dam was generally held in place during casting by a side dam holder. The side dam typically included a V-shaped beveled bottom portion and the side dam holder typically included a V-shaped receptacle into which the V-shaped beveled bottom portion of the side dam was seated. The V-shape configuration served to position and hold the side dam in place during casting. However, such side dam assemblies limited the useful life of the side dams before causing serious damage to the casting equipment as well as adversely impacting the edges of the cast strip. Specifically, the degree of side dam wear had to be limited to prevent the clashing of the side dam holder V shaped receptacle with the casting roll edge, limiting the service life of the side dam. Therefore, the side dams were always replaced before such damage to casting equipment could occur, limiting the duration of the casting campaign. As explained above, when the side dams were changed, the removable tundish and core nozzle were generally also changed and a new casting campaign started. The casting costs per ton of thin strip cast thus could be considerably reduced if the useful life of the side dams could be extended.
In summary, no matter which refractory component has worn out first, a casting campaign will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life as a precaution to avoid further disruption of the next casting campaign. This results in attendant waste of useful life of refractory components.
Further limitations and disadvantages of previously used and proposed thin strip casting systems and methods will become apparent to one of skill in the art, through comparison of such systems and methods with the present invention as set forth in this present application.